Where does CRC-24 actually show up?

Its best-known real-world use is RFC 4880's OpenPGP ASCII-armor format: every armored PGP/GPG message, key, or signature block ends with a line starting with '=' followed by 4 base64 characters, which decode to exactly this 24-bit CRC-24/OpenPGP checksum of the preceding binary data. If you've ever seen a '-----BEGIN PGP MESSAGE-----' block, that trailing '=' line is what this tool computes (for the raw text, not the OpenPGP binary packet itself).

What are this checksum's exact parameters?

Polynomial 0x1864CFB, initial value 0xB704CE, no input or output reflection, and no final XOR, exactly as specified in RFC 4880 Appendix B. Like CRC-8 elsewhere in this category, it's a non-reflected (MSB-first) CRC, unlike the reflected CRC-16 and CRC-32 tools.

Why is CRC24 output 6 hex characters?

CRC-24 always produces a 24-bit result regardless of input size, and 24 bits written as hex is exactly 6 characters (24 / 4 = 6). That sits between CRC-16's 4 characters and CRC-32's 8, chosen by OpenPGP's designers as a reasonable balance for detecting transmission errors in armored text without much overhead.

Does this tool send my text to a server?

No. The CRC-24 checksum is computed entirely in your browser using a hand-written, table-driven TypeScript implementation, so nothing you type here is transmitted anywhere.

CRC24 Calculator - Compute CRC-24/OpenPGP Checksum Online

Overview

Introduction

CRC-24 has one dominant real-world use: it's the checksum appended to every ASCII-armored OpenPGP message, signature, and key block, defined precisely in RFC 4880 Appendix B.

This tool computes that exact CRC-24/OpenPGP checksum of any text you provide, entirely client-side, useful for verifying an armor checksum by hand or reproducing the RFC's own worked example.

What Is CRC24 Calculator?

CRC-24 is a 24-bit cyclic redundancy check, a polynomial-division-based checksum for catching accidental transmission errors, shown here as 6 hexadecimal characters.

This tool implements CRC-24/OpenPGP specifically: polynomial 0x1864CFB, initial value 0xB704CE, no reflection, no final XOR, exactly as RFC 4880 defines it for ASCII-armored PGP data.

How CRC24 Calculator Works

Your UTF-8 encoded text is processed one byte at a time through a lookup table built from the 0x1864CFB polynomial, most-significant-bit first, starting from the RFC-specified 0xB704CE initial value rather than zero.

Each byte updates a 24-bit running remainder via table lookup and XOR; the final remainder, with no reflection or additional XOR, is hex-encoded into the 6-character output this tool returns.

When To Use CRC24 Calculator

Use CRC-24 specifically when reproducing or verifying an OpenPGP ASCII-armor checksum line by hand, or implementing/debugging PGP-armor-compatible software.

Don't use CRC-24 for general file integrity or anywhere security matters; like all CRCs it offers no resistance to deliberate tampering, only accidental-error detection.

Often used alongside CRC8 Calculator, CRC16 Calculator and CRC32 Calculator.

Features

Advantages

Exactly matches a widely deployed real-world standard (OpenPGP armor), so there's no ambiguity about which parameter set to use, unlike CRC-8 where dozens of variants exist.
24-bit output balances overhead against error-detection strength for text-based armored data.
Simple, table-driven computation that's fast even for larger pasted PGP-armored blocks.

Limitations

Essentially single-purpose: outside OpenPGP armor, CRC-24 sees little other real-world use compared to CRC-16 or CRC-32.
Only catches accidental corruption, not deliberate tampering, an attacker who can modify armored data can trivially recompute a matching checksum.
This tool checksums raw text you paste in, not an actual binary OpenPGP packet stream, so it won't directly validate a real .asc file's trailer without extracting the right underlying bytes.

Examples

Hashing the standard CRC check string

Input

123456789

Output

21cf02

The official CRC-24/OpenPGP check value for the ASCII string "123456789", the standard sanity-check input used across the reveng CRC catalogue.

Hashing a short greeting

Input

Hello, world!

Output

1bdf82

13 bytes of UTF-8 text reduced to the 6-character CRC-24/OpenPGP checksum.

Best Practices & Notes

Best Practices

Use CRC-24 specifically for OpenPGP-armor-related work; for any other checksum need, CRC-16 or CRC-32 are the more commonly expected defaults.
Remember this computes a checksum over raw text, not a full OpenPGP binary packet, when comparing against a real .asc file's armor line.
Never treat a matching CRC-24 as proof of anything beyond accidental-corruption-free transmission.

Developer Notes

Implemented as a table-driven, non-reflected (MSB-first) CRC using polynomial 0x1864CFB and initial value 0xB704CE, matching RFC 4880 Appendix B exactly. Verified against the official CRC-24/OpenPGP check value (0x21cf02 for the ASCII string "123456789", the standard reveng CRC catalogue sanity check) plus an independently confirmed vector for "Hello, world!".

CRC24 Calculator Use Cases

Reproducing an OpenPGP ASCII-armor checksum line by hand while studying RFC 4880
Debugging a PGP/GPG-armor-compatible implementation's CRC-24 computation
Confirming a CRC-24 implementation matches the standard OpenPGP check value
Teaching how ASCII-armored PGP messages self-check for transmission corruption

Common Mistakes

Assuming CRC-24 starts from a zero initial value like CRC-32 does; OpenPGP's variant starts from 0xB704CE specifically.
Expecting this tool's output to directly match a real .asc file's armor line without accounting for the difference between raw text and the actual OpenPGP binary packet bytes.
Treating a matching CRC-24 as any kind of security guarantee rather than accidental-corruption detection.

Tips

If you're validating a real PGP-armored file, extract and base64-decode the actual binary packet data before computing CRC-24 over it, not the armored text as displayed.
For a general-purpose (non-OpenPGP-specific) checksum, this category's CRC-16 or CRC-32 tools are the more broadly recognized choice.

CRC24 Calculator